The use of semi-permeable membranes to separate gas mixtures has become a well known technique in the production of industrial gases. Known plants for the separation of gas mixtures, for example, air, by such membranes are constructed so as to present a large surface area of membrane to the air to be separated.
For example, such plants may employ a multitude of identical, elongate, hollow fibres which are formed from a suitable semi-permeable membrane and which extend in parallel to one another. The fibres are usually mounted in and typically extend from one end to the other of a pressure vessel. The air to be separated is fed under pressure into a header at or adjacent one end of the fibres so that it flows longitudinally through the fibres. The insides of the fibres are maintained at a higher pressure than that which obtains on the outside of the fibres. The components of the air diffuse through the membranes at different speeds such that as the air passes along the inside of a membrane so a faster permeating component, for example, oxygen, passes more and more to the low pressure side. Accordingly, a nitrogen rich product gas in the order of 99.5% by volume nitrogen may be withdrawn under pressure from the end of the pressure vessel opposite that at which the feed gas is introduced.
It is known that water vapour has no deleterious effect on the membranes and in fact the membranes are effective at removing water vapour from the nitrogen product gas stream.
However, free water tends to block the membranes pores because the fibre ends are blocked irreversibly by capillary action.
In order to overcome this problem it is known to incorporate in a gas separation apparatus a coalescing filter upstream of the pressure vessel containing the membranes. This produces saturated air which, if allowed to cool, will cause water condensation. Therefore, a preheater to the air feed is often incorporated also.
In practice this preheater only operates when the compressor is operating and there is frequently a danger of free water hitting the membranes before the system has reached a steady state temperature.
It is an aim of the present invention to remove this danger by incorporating in a gas separation apparatus a regenerative desiccant.